As a turbocharger mountable on a diesel engine, there has been conventionally known a variable geometry turbocharger having a nozzle through which exhaust gas is ejected and whose opening degree is adjustable. An opening degree adjustment mechanism for the nozzle is installed in the variable geometry turbocharger. It has been suggested to use a hydraulic actuator to actuate such an opening degree adjustment mechanism (e.g., Patent Literature 1).
Some diesel engines employ exhaust gas recirculation (hereinafter abbreviated as “EGR”), so that exhaust gas is partly returned back to the air-intake side in order to lower a combustion temperature to suppress emission of NOx. An arrangement for realizing EGR generally includes an EGR pipe that communicates an exhaust pipe and an intake pipe and an EGR valve device provided in the middle of the EGR pipe. It has also been suggested to use a hydraulic actuator to control the opening and closing of the EGR valve device (e.g., Patent Literature 2).
A hydraulic actuator for a variable geometry turbocharger or an EGR device uses pilot pressure generated by an electronic proportional control valve (hereinafter abbreviated as “EPC valve”) to move a pilot spool to a position where the pilot spool is balanced with a spring that provides reaction force. The hydraulic actuator also uses pump pressure oil from a main pump (hereinafter referred to as “main pump pressure oil”) to move a hydraulic piston following the pilot spool, thereby, in conjunction with the hydraulic piston, actuating the opening degree adjustment mechanism in the variable geometry turbocharger or controlling the operation of the EGR valve. Specifically, by adjusting an electric current provided to the EPC valve, it is possible to adjust the stop position of the spool movable with the pilot pressure corresponding to the electric current. With this arrangement, in case of the variable geometry turbocharger, the movement amount of the hydraulic piston and the resulting opening degree of the nozzle determined through the opening degree adjustment mechanism become controllable. In case of the EGR device, the opening degree of a poppet valve in an area through which exhaust gas passes becomes controllable.
Typically, when an engine is provided with not only a variable geometry turbocharger as disclosed in Patent Literature 1 but also an EGR valve device as disclosed in Patent Literature 2, the pressure oil from the main pump is temporarily directed into a hydraulic manifold block, and is divided therein into pressure oil for the variable geometry turbocharger and pressure oil for the EGR valve.
Since the temperature of the variable geometry turbocharger can become considerably high, a hydraulic actuator for the variable geometry turbocharger is attached to the variable geometry turbocharger. In contrast, the EPC valve, which is easily affected by heat, is attached not to the variable geometry turbocharger or the hydraulic actuator but to the hydraulic manifold block, which is located at a position free from the thermal influence of the variable geometry turbocharger. Since exhaust gas passes through the EGR valve device after being cooled by an EGR cooler, the temperature of a solenoid actuator for the EGR valve device does not become so high. Thus, the solenoid actuator is installed as a part of the EGR valve device.
Citation List
Patent Literature
Patent Literature 1: WO 08/053771 A1
Patent Literature 2: WO 05/095834 A1